This invention relates to variable delivery compressors of the multicylinder/double acting type, and more particularly to compressors of this kind which are highly efficient with minimum loss of power during reduced delivery operation.
Compressors for use in air conditioning systems for vehicles are usually driven by engines installed on the vehicles, and have their total displacement or delivery quantity set at values suitable to ensure that the temperature within the compartment of a vehicle, which has risen to a high value during parking under the scorching sun, can be lowered promptly after the start of the engine of the vehicle. As a result, when the engine operation continues at a mean speed, for instance at a cruising speed for some period of time after the start of the engine, the cooling capacity of such a compressor can increase to an excessive degree. Therefore, when the cooling capacity of the compressor increases to an excessive degree during high speed operation of the engine, it is necessary to control the delivery quantity of the compressor to a smaller value for economy of power as well as for prevention of overcooling. Also when there occurs a drop in the cooling load on the air conditioning system, the delivery quantity of the compressor should be controlled to a smaller value, for the same purposes as above. Conventionally, as a simple means for varying the delivery quantity of such a compressor, a step control method is known which renders part of the cylinders of the compressor substantially ineffective by bypassing their discharge side to their suction side.
However, if such a step control method is applied to a multicylinder/double acting type compressor, compression of compression fluid or refrigerant and discharge of same through the discharge valves are continuously carried out on the side of the substantially ineffective cylinders due to reciprocating motions of the pistons within these cylinders. That is, during such reduced delivery operation, the power consumption is not reduced in proportion to the number of cylinders rendered ineffective, and is still larger for the number of the other effective cylinders. Further, due to discharge resistance of the discharge valves, the pressure and temperature of compression fluid discharged from the ineffective cylinders increase, and this compression fluid is sucked into the other effective cylinders, resulting in an increase in the temperature of the discharge fluid.